Make library generation unit tests a required check

[JoeWang1127]

The library-generation-unit-tests sprouts two jobs: library-generation-unit-tests (8, macos-12) and library-generation-unit-tests (8, ubuntu-22.04).

The behavior of this check when setting library-generation-unit-tests (the parent job) as required:

• On pull request that doesn't change library_generation directory: checks initiate and return result.
• On pull request that changes library_generation directory: library-generation-unit-tests is stuck.

The behavior of this check when setting two child jobs as required:

• On pull request that doesn't change library_generation directory: library-generation-unit-tests (8, macos-12) and library-generation-unit-tests (8, ubuntu-22.04) are stuck.
• On pull request that changes library_generation directory: checks initiate and return result.

The behavior of this check when setting parent and two child jobs as required:

• On pull request that doesn't change library_generation directory: library-generation-unit-tests (8, macos-12) and library-generation-unit-tests (8, ubuntu-22.04) are stuck.
• On pull request that changes library_generation directory: library-generation-unit-tests is stuck.

[JoeWang1127]

A potential solution: remove unit tests on macos since it adds little value.

[suztomo]

Required checks that only run on certain path changes are discussed in https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/collaborating-on-repositories-with-code-quality-features/troubleshooting-required-status-checks#handling-skipped-but-required-checks. I don't see we use the workaround.